The present invention relates to blends of phase change materials and clay matrices, methods of making them, and devices using the blends.
Phase change materials (PCMs) are well known materials that utilize their latent heat of fusion to absorb, store, and release thermal energy during phase conversions between solid and liquid phases. In particular, a PCM absorbs/releases a large quantity of energy in the vicinity of its melting/freezing point, i.e. phase change temperature, however, the temperature of the substance itself remains about the same. These latent heats of fusion are greater than the sensible heat capacities of the materials. More specifically, upon melting and freezing, per unit weight, a PCM absorbs and releases substantially more energy than a sensible heat storage material that is heated or cooled to the same temperature range. The latent heat of fusion of many PCMs is substantial and can be used as a heat source or a heat sink. Thus, various PCMs have been applied in building structures, solar collectors, thermal energy storage (TES) units, road base materials, beverage and food containers, orthopedic devices, medical wraps, and textile applications such as garments.
For example, U.S. Pat. No. 4,711,813 to Salyer, discloses a polyethylene composite formed from cross-linked polyethylene having a straight chain (crystalline) alkyl hydrocarbon incorporated therein as a phase change material which may be manufactured as pellets or in sheet form. The polyethylene composite is useful for incorporation into concrete or other building materials and is used for wall or floor coverings, fire retardants, or runway, roadway or bridge de-icing, and the like.
U.S. Pat. No. 4,617,332 to Salyer describes a phase change composition comprising a matrix material selected from the group consisting of hydraulic cement, gypsum, lime, and plaster of paris, and a plurality of pellets or granules containing a crystalline straight chain alkyl hydrocarbon or a mixture of crystalline, straight chain, alkyl hydrocarbons with the hydrocarbons having at least 14 carbon atoms and a heat of fusion greater than 30 cal/g.
U.S. Pat. No. 4,504,402 to Chen et al. discloses encapsulated phase change materials in pellets for incorporation into concrete or other building materials. The encapsulated phase change compositions may be a Glauber salt eutectic mixture, sodium hydroxide, polyethylene, sodium sulfate decahydrate, sodium thiosulfate pentahydrate, calcium chloride hexahydrate, magnesium nitrate hexahydrate, the eutectic of magnesium nitrate hexahydrate and ammonium nitrate, potassium fluoride tetrahydrate, sodium acetate trihydrate, stearic acid, the eutectic of naphthalene and benzoic acid, and paraffinic hydrocarbons. Shell materials described in the patent include a copolymer latex of butadieneacrylonitrile, a copolymer of vinylidene chloride-acrylic, resinous latexes, rubber latexes, epoxy polymers, polyurethane polymers, acrylic polymers, cellulose acetate, and polyamides.
U.S. Pat. No. 4,708,812 to Hatfield discloses encapsulated phase change material particles, such as a crystalline polymer, naphthalene, salt hydrate, and a crystalline paraffin.
U.S. Pat. No. 4,587,279 to Salyer describes a composition comprising an inorganic building material and an end-capped polyethylene glycol dispersed therein as a phase change material.
One of the basic problems, however, with the use of solid-to-liquid PCMs for temperature control, is containment. In particular, for solid-to-liquid PCMs, the PCM exists as a solid phase PCM below the PCM melting point, in the form of a thick block or agglomeration, and as a liquid phase PCM above the PCM melting point. Particular problems are associated with the presence of a liquid phase PCM, such as leakage.
U.S. Pat. Nos. 5,211,949 and 5,106,520, both to Salyer, disclose a conformable, powder-like mix of silica particles having a critical size of about 0.007 to about 0.07 microns, as a matrix, and a PCM. The mix does not tend to solidify at temperatures below the PCM melting point and does not liquefy at temperatures above the PCM melting point. However, the spherical silica particles are relatively expensive, difficult to process, and are very hygroscopic, necessitating special packaging materials.
Accordingly, there is still a need for a phase change material blend using an economical and readily available matrix and that can be provided in a loosely bound or powder-like form. The present invention is directed to overcoming these and other deficiencies in the art.
The present invention relates to a powdered mix of a phase change material blend. The blend includes a polar phase change material bound substantially to at least one surface of a clay matrix having a surface charge, wherein the blend is present in a powdered form at temperatures above and below the phase change temperature of the polar phase change material.
The present invention also relates to a method of making a powdered mix of a phase change material blend. The method includes mixing a clay matrix having a surface charge with a polar phase change material under conditions effective to bind the polar phase change material substantially to at least one surface of the clay matrix in a powdered form at temperatures above and below the phase change temperature of the polar phase change material.
Another embodiment of the present invention relates to a therapy pack including an outer material encapsulating a powdered mix of a phase change material blend, wherein the phase change material blend includes a polar phase change material bound substantially to at least one surface of a clay matrix having a surface charge and wherein the blend is present in a powdered form at temperatures above and below the phase change temperature of the polar phase change material.
The phase change material blend of the present invention utilizes a clay matrix as a substrate for a polar phase change material (PCM) that remains loosely bound or powder-like, at temperatures above and below the phase change temperature of the PCM. In particular, the clay matrix-PCM blend will not liquefy upon heating of the PCM above its melting point and will not form a rigid solid at temperatures below the melting point of the PCM (i.e., the blend is powder-like throughout the temperature range). The PCM can change phase while bound to the clay matrix and can utilize its latent heat of fusion to absorb, store, and release heat or cool during such phase conversions. The blend of the present invention uses an economical, readily available, and naturally occurring matrix for the PCM. In addition, the consistency of the blend can be varied according to the type of clay matrix used and the size of the particles in the clay matrix.